I've gone through the usual steps of cleaning the tracks and spiral springs, then lubricating them with a silicone spray, paying particular attention to the springs. Still too hard to raise from the fully closed position. Fairly easy to raise once the sash is open about three inches. The sash maintains its position when I take my hands off it, implying that the springs are under proper tension.
I removed the lower sash. Tilting the sash inward to remove it causes the shoes at the bottom ends of the springs to expand and wedge into the sides of the track, so the shoes don't snap upwards. This is explained at time 1:46 here:
I then used a screwdriver to twist the center of the shoe 90 degrees, which releases it and retracts the spring. If I now use the screwdriver to force the shoe all the way down, it remains in the down position even when I remove the screwdriver, instead of snapping upward. Once I move the shoe about three inches above the fully down position, it snaps upward as it should.
I can't understand why the shoes (both sides) are so tight when fully down. I used plenty of lubricant on all three sides of each shoe. Any ideas?
Thanks for your insights and suggestions.
R1